1. Field of the Invention
This invention relates to a process for the production of a thin film and more particularly, it is concerned with a novel process for the production of a multicomponent amorphous silicon thin film.
2. Description of the Prior Art
For the production of a thin film, there have hitherto been proposed PVD (Physical Vapor Deposition) methods such as vacuum vapor deposition method, ion plating method and sputtering method, CVD (Chemical Vapor Deposition) methods such as ordinary pressure CVD method, reduced pressure CVD method, plasma CVD method and spraying method, and plating methods. Above all, the sputtering method and plasma CVD method have lately been used widely in the field of manufacturing thin films, since they make possible production of thin films which has hitherto been impossible, and they make possible production of thin films at low temperature by utilizing energy of glow discharge plasma.
The properties required for thin films differ depending on the quality and use thereof and a process for the production thereof is correspondingly limited. Problems of the production process will be discussed as to amorphous silicon exemplified, which has lately been watched as a low cost thin film semiconductor material for a solar cell capable of utilizing clean and inexhaustible energy. The plasma CVD method, sputtering method, ion plating method, ion beam sputtering method and CVD method have been studied as a method of making an amorphous silicon film, and at the present time, the plasma CVD method has been considered best from the standpoint of the film quality, i.e. device properties as, for example, solar cell.
When an amorphous silicon film is produced by the plasma CVD method, however, the film growing rate is very low, i.e. at most about 1 .ANG./sec and thus the production cost is not so decreased. Furthermore, solar cells of amorphous silicon have a lower conversion efficiency than those of other crystal semiconductors such as Si and GaAs. Therefore, amorphous silicon has not been used yet as a power source.
In order to increase the photovoltaic conversion efficiency, effective use of solar energy is required over wide wavelength range and to this end, it has been studied to add other elements (modifiers) to amorphous silicon to vary the optical band gap thereof. However, this method has a drawback that in the case of adding a modifier to amorphous silicon, the modifier should be used in gaseous state according to the plasma CVD method, so that a raw material gas is limited.
Moreover, it has been proposed to use an amorphous silicon film fabricated by a process of a glow discharge in silane as a body of semiconductor devices (U.S. Pat. No. 4,064,521). The thus resulting amorphous silicon has a greater electron life time than amorphous silicon formed by the sputtering or evaporation method, but this process is not suited for incorporating modifiers into amorphous silicon.